Home » Public Forums » GMCnet » My thoughts on the rear ride height (For your consideration...)
| My thoughts on the rear ride height [message #359175] |
Sun, 04 October 2020 07:38  |
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RF_Burns
Messages: 2277
Registered: June 2008
Location: S. Ontario, Canada
Karma: 3
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The GMC ride heights are set so the rear sits about an inch lower than the front. This is my take on why this is so.
I don't believe it has anything to do with getting maximum caster. An inch change in the rear height only changes the front caster by 1/3 degree. This isn't up for debate, its simple math. Caster recommendations are all over the place, but most agree more caster the better up to about 4 degrees or so.
My thoughts are the rear height has to do with stability of the rear bogie arms. When the rear is set to the recommended ride height, the rear axles (if I am allowed to call them axles) sit about an inch higher than the bogie pins. I'm thinking with the bogie pins sitting lower than the wheel axles, the bogie arms are more directionally stable, like laying in a hammock. The bogie pins carry the rear weight of the coach. My senses tell me that the rear suspension is more stable when the bogie pins are below the wheel axles.
When you raise the rear, the bogie pins become level or above the wheel axles. Once the bogie pins are close to level or above the wheel axles, the bogie arms (especially the front arm) become directionally unstable which makes the rear of the coach squirrely. The higher the bogie pins are above the wheel axles, the more directionally unstable the bogie arms become (to a certain point).
If one was to mount the bogies lower on the frame (with the necessary frame work), you could raise the rear of the coach while maintaining the lower bogie pin configuration and maintain the desired stability.
Just my farmboy mechanic way of seeing things.
What say yeh?
Bruce Hislop
ON Canada
77PB, 455 Dick P. rebuilt, DynamicEFI EBL EFI & ESC.
1 ton front end
http://www.gmcmhphotos.com/photos/showphoto.php?photo=29001
My Staff says I never listen to them, or something like that
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| Re: My thoughts on the rear ride height [message #359182 is a reply to message #359175] |
Sun, 04 October 2020 09:06  |
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Richard RV
Messages: 631
Registered: July 2012
Location: Full-timer for 12 years, ...
Karma: -17
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Bruce,
Seems reasonable. Hal St Clair is using taller frame rails and it looks like he dropped the bogey box with some square tubing under the frame rail. I'd like his input.
Richard
'77 Birchaven TZE...777;
'76 Palm Beach under construction;
‘76 Edgemont waiting its turn
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| Re: My thoughts on the rear ride height [message #359184 is a reply to message #359182] |
Sun, 04 October 2020 09:45 |
mghamms
Messages: 466
Registered: March 2016
Location: Ware, Massachusetts
Karma: 2
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I have to agree
I tested the handling by raising and lowering the rear,
Squirrely when the rear is raised. The rear was all over the place.
Level was much better and acceptable.
Slightly lower was the best.
As for how much your chose. Just be safe for yourself and others.
1977 Kingsley 455 as stock as it gets except lots of Ragusa parts
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| Re: My thoughts on the rear ride height [message #359185 is a reply to message #359175] |
Sun, 04 October 2020 09:46 |
Carl S.
Messages: 4186
Registered: January 2009
Location: Tucson, AZ.
Karma: 13
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Whenever I exit the freeway or slow down below about 45 in a town, I set my Power Level II valves to full raise and leave them there so I don't drag the back end over uneven intersections or when entering/leaving driveways, etc. I have found myself forgetting to return the valves to the travel position on occasion, only noticing it miles down the road.
Interestingly, I have never really noticed a difference in driveability or stability while driving down the road with the back end up. I generally change it to travel when I notice my mistake, but it doesn't seem to make any difference.
Carl Stouffer
'75 ex Palm Beach
Tucson, AZ.
Chuck Aulgur Reaction Arm Disc Brakes, Quadrabags, 3.70 LSD final drive, Lenzi knuckles/hubs, Dodge Truck 16" X 8" front wheels, Rear American Eagles, Solar battery charging. GMCSJ and GMCMI member
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| Re: My thoughts on the rear ride height [message #359189 is a reply to message #359175] |
Sun, 04 October 2020 11:06 |
JohnL455
Messages: 4447
Registered: October 2006
Location: Woodstock, IL
Karma: 12
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The greater the arc, the shorter the lead/ lag length, from spindle to pin.
However, when at correct ride height, there seems to be an imaginary line connected through the four points determined through the spindle, pin, pin, and spindle. Your theory would produce a U shaped arc.
John Lebetski
Woodstock, IL
77 Eleganza II
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| Re: [GMCnet] My thoughts on the rear ride height [message #359191 is a reply to message #359189] |
Sun, 04 October 2020 11:45 |
James Hupy
Messages: 6806
Registered: May 2010
Karma: -62
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I have not heard one concern or comment from this esteemed collection of
GMC enthusiasts about the real reasons that the GMC engineers had for not
setting the frame level. A "Gibbs slap" to all of you that failed to
recognize anti-dive geometry when it is right in front of your faces.
Assume if you will that directly under the rear frame slot on the ground is
the baseline of a triangle. That baseline extends forward to a spot on the
ground directly under the front frame slot. From there, go directly
vertical at 90° through the front slot. Then scribe the third line of this
triangle back to the spot on the ground under the rear frame slot. Thus
completing the tri-angle of the anti-dive concept.
When heavy braking is applied to the GMC suspension, this fat end of
the tri-angle performs the task of preventing nose dive of the front end,
and unloading of the rear/rear tires on the road surface, helping with the
prevention of flat spotting those expensive tires, and subsequent loss of
control, although slight. So, measure anything you choose, grease fittings,
bottom of the frame, middle of the slots, or any point on the bodywork of
your choosing.
"JUST DO NOT CHANGE THE DIMENSIONS OF THAT ANTI -DIVE TRIANGLE", and
you should be fine.
Thats my story, and I'm sticking to it.
Jim Hupy
Salem, Oregon
On Sun, Oct 4, 2020, 9:07 AM John R. Lebetski via Gmclist <
gmclist@list.gmcnet.org> wrote:
> The greater the arc, the shorter the lead/ lag length, from spindle to
> pin.
> However, when at correct ride height, there seems to be an imaginary line
> connected through the four points determined through the spindle, pin, pin,
> and spindle. Your theory would produce a U shaped arc.
> --
> John Lebetski
> Woodstock, IL
> 77 Eleganza II
>
>
> _______________________________________________
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| Re: [GMCnet] My thoughts on the rear ride height [message #359192 is a reply to message #359191] |
Sun, 04 October 2020 11:58 |
Ken Henderson
Messages: 8726
Registered: March 2004
Location: Americus, GA
Karma: 9
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Jim,
If the GMC engineers really did use the "anti-dive triangle" as you
described, you'd think they'd have been smart enough to realize that the
whole thing would be blown out of the water by the "pole-vaulting"
over those dumb center wheels! Those "expensive rear tires" don't seem to
have counted for much in that decision. :-)
Ken H.
On Sun, Oct 4, 2020 at 12:46 PM James Hupy via Gmclist <
gmclist@list.gmcnet.org> wrote:
> I have not heard one concern or comment from this esteemed collection of
> GMC enthusiasts about the real reasons that the GMC engineers had for not
> setting the frame level. A "Gibbs slap" to all of you that failed to
> recognize anti-dive geometry when it is right in front of your faces.
> Assume if you will that directly under the rear frame slot on the ground is
> the baseline of a triangle. That baseline extends forward to a spot on the
> ground directly under the front frame slot. From there, go directly
> vertical at 90° through the front slot. Then scribe the third line of this
> triangle back to the spot on the ground under the rear frame slot. Thus
> completing the tri-angle of the anti-dive concept.
> When heavy braking is applied to the GMC suspension, this fat end of
> the tri-angle performs the task of preventing nose dive of the front end,
> and unloading of the rear/rear tires on the road surface, helping with the
> prevention of flat spotting those expensive tires, and subsequent loss of
> control, although slight. So, measure anything you choose, grease fittings,
> bottom of the frame, middle of the slots, or any point on the bodywork of
> your choosing.
> "JUST DO NOT CHANGE THE DIMENSIONS OF THAT ANTI -DIVE TRIANGLE", and
> you should be fine.
> Thats my story, and I'm sticking to it.
> Jim Hupy
> Salem, Oregon
>
> On Sun, Oct 4, 2020, 9:07 AM John R. Lebetski via Gmclist gmclist@list.gmcnet.org> wrote:
>
>> The greater the arc, the shorter the lead/ lag length, from spindle to
>> pin.
>> However, when at correct ride height, there seems to be an imaginary line
>> connected through the four points determined through the spindle, pin,
> pin,
>> and spindle. Your theory would produce a U shaped arc.
>> --
>> John Lebetski
>> Woodstock, IL
>> 77 Eleganza II
>>
>>
>> _______________________________________________
>> GMCnet mailing list
>> Unsubscribe or Change List Options:
>> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
>>
> _______________________________________________
> GMCnet mailing list
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Ken Henderson
Americus, GA
www.gmcwipersetc.com
Large Wiring Diagrams
76 X-Birchaven
76 X-Palm Beach
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| Re: [GMCnet] My thoughts on the rear ride height [message #359196 is a reply to message #359193] |
Sun, 04 October 2020 16:03 |
Ken Henderson
Messages: 8726
Registered: March 2004
Location: Americus, GA
Karma: 9
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Do what!? It doesn't take extraordinary adhesion of the tires to cause
pole vaulting -- it happens EVERY time the brakes are applied on the center
wheels. It HAS to in order to counteract the braking forces, no matter how
slight! And if you smoked 6 tires, you did indeed have adhesion; perhaps
not as much as with radials, but certainly enough to cause significant pole
vaulting.
Ken H.
On Sun, Oct 4, 2020 at 2:27 PM Matt Colie via Gmclist <
gmclist@list.gmcnet.org> wrote:
> Ken Henderson wrote on Sun, 04 October 2020 12:58
>> Jim,
>>
>> If the GMC engineers really did use the "anti-dive triangle" as you
> described, you'd think they'd have been smart enough to realize that the
> whole
>> thing would be blown out of the water by the "pole-vaulting" over those
> dumb center wheels! Those "expensive rear tires" don't seem to have counted
>> for much in that decision. :)
>>
>> Ken H.
>
> Ken,
>
> You seem to have overlooked the fact that all design work was done by the
> time radial tires came on the general scene. Tires were very different back
> then. They were cheaper, but nothing got the kind of tire life we see
> today. The "pole-vaulting" you speak of simply could not occur with the
> tires
> of the day. The effective traction was not available to do it.
>
> When I panic stopped Frank Sargent's brand new coach while ferrying back
> from Pontiac, it smoked all six tires. The LEO that was observing what he
> expected to be a crash was impressed that there was nothing to write up
> except the bozo that had pulled out in front of me was clearly in
> violation.
> I told the office about it when I turned in the keys and never heard if
> they sent to coach to have the tires trued.
>
> Matt
> --
> Matt & Mary Colie - Chaumière -'73 Glacier 23 - Members GMCMI, GMCGL,
> GMCES
> Electronically Controlled Quiet Engine Cooling Fan with OE Rear Drum
> Brakes with Applied Control Arms
> SE Michigan - Near DTW - Twixt A2 and Detroit
>
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Ken Henderson
Americus, GA
www.gmcwipersetc.com
Large Wiring Diagrams
76 X-Birchaven
76 X-Palm Beach
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| Re: My thoughts on the rear ride height [message #359198 is a reply to message #359185] |
Sun, 04 October 2020 17:34 |
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Richard RV
Messages: 631
Registered: July 2012
Location: Full-timer for 12 years, ...
Karma: -17
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Carl, the fact that you don't notice a difference in handling with changes in ride height tells me your alignment is good. I think the difference is also more noticeable with the OEM front end as it has less caster.
Richard
'77 Birchaven TZE...777;
'76 Palm Beach under construction;
‘76 Edgemont waiting its turn
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| Re: My thoughts on the rear ride height [message #359199 is a reply to message #359198] |
Sun, 04 October 2020 17:45 |
Carl S.
Messages: 4186
Registered: January 2009
Location: Tucson, AZ.
Karma: 13
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Senior Member |
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Richard,
My front end IS OEM (Lensi hubs, knuckles, bearings and urethane bushings). The only thing I amm doing different from most is the 8" wide slightly different offset wheels. But yes, you're right. My alignment is good. My front tires are almost to the wear bars with totally even wear on them. My wife and I were just discussing the need to replace them (very) soon.
Carl Stouffer
'75 ex Palm Beach
Tucson, AZ.
Chuck Aulgur Reaction Arm Disc Brakes, Quadrabags, 3.70 LSD final drive, Lenzi knuckles/hubs, Dodge Truck 16" X 8" front wheels, Rear American Eagles, Solar battery charging. GMCSJ and GMCMI member
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| Re: [GMCnet] My thoughts on the rear ride height [message #359201 is a reply to message #359199] |
Sun, 04 October 2020 21:26 |
johnd01
Messages: 354
Registered: July 2017
Location: Sacrameot
Karma: -1
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Wear bars are almost unheard of on this coach or any other motor home.
On Sun, Oct 4, 2020 at 3:46 PM Carl Stouffer via Gmclist <
gmclist@list.gmcnet.org> wrote:
> Richard,
>
> My front end IS OEM (Lensi hubs, knuckles, bearings and urethane
> bushings). The only thing I amm doing different from most is the 8" wide
> slightly
> different offset wheels. But yes, you're right. My alignment is good.
> My front tires are almost to the wear bars with totally even wear on them.
> My wife and I were just discussing the need to replace them (very) soon.
> --
> Carl Stouffer
> '75 ex Palm Beach
> Tucson, AZ.
> Chuck Aulgur Reaction Arm Disc Brakes, Quadrabags, 3.70 LSD final drive,
> Lenzi knuckles/hubs, Dodge Truck 16" X 8" front wheels, Rear American
> Eagles,
> Solar battery charging. GMCSJ and GMCMI member
>
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
>
--
*John Phillips*
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Johnd01
John Phillips
Avion A2600 TZE064V101164
Rancho Cordova, CA (Sacramento)
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| Re: My thoughts on the rear ride height [message #359202 is a reply to message #359189] |
Sun, 04 October 2020 21:56 |
78 Barbi
Messages: 80
Registered: August 2004
Location: Tiverton, Ontario , Canad...
Karma: -1
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John writes : An imaginary line connected through the four points determined through the spindle, pin, pin, and spindle. Your theory would produce a U shaped arc. I was at a rally years ago and the " Panel of
GMC Experts " were having a discussion on the stage regarding GMC ride height " and one of the experts proclaimed that when the zerks and the rear wheel centres were in a straight line , then the ride
height was correct .
This is complete BS, there will definitely be an " U arc " between the wheel centers and the zerks. Our coach is presently sitting on blocks placed beneath the bogie boxes and these blocks were installed with the frame at maximum height. Just went over and measured the distance from the concrete pad to the zerk centre line , it was 12-7/8" , the rolling radius of a GMC wheel / tire assembly is 13-5/8" so yes once again , at proper ride height the zerks and wheel centers are NOT in a straight line. OPINIONS may vary but the actual measurements are quite sound. Reminds me of the " wag " a while back
commenting on our Kelsey Hayes disc parking brake . He mentioned it was " marginally effective " , well a parking brake that was tested to a 13% grade is a very effective parking brake , in fact it has a higher clamp force mechanically than it does hydraulically, I seriously doubt the guy was visually impaired or mentally impaired so that only left ignorance, wilful ignorance at that ! RANT over !
[Updated on: Sun, 04 October 2020 22:55] Report message to a moderator |
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| Re: My thoughts on the rear ride height [message #359224 is a reply to message #359175] |
Tue, 06 October 2020 08:53 |
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RF_Burns
Messages: 2277
Registered: June 2008
Location: S. Ontario, Canada
Karma: 3
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Senior Member |
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I must apologize for I made an error in my original posting.
For the 1/3 degree caster change, I used the 160" wheel base from the front wheels to the bogie mount. But the rear slot where measurements are made is another 54.5" behind the bogie mount, so I should have used 214.5".
Using 214.5" as the base line of a right angle triangle and one inch as the rise at the right angle, the far angle would be 0.26 degrees. Therefore raising the rear of the coach 1 inch as measured at the rear slot would decrease the caster by 0.26 degrees or about 1/4 of a degree.
I believe the uncertainties and inaccuracies in caster measurement, plus the mechanical flex, give and clearances of the bushings, balljoints and A arms certainly add up to more than 1/4 degree, so this change becomes immaterial in my farmboy mechanic view.
So I'm still thinking the bogie pins below the rear axles give more stability to the rear suspension and that is what we experience.
Fire away!
Bruce Hislop
ON Canada
77PB, 455 Dick P. rebuilt, DynamicEFI EBL EFI & ESC.
1 ton front end
http://www.gmcmhphotos.com/photos/showphoto.php?photo=29001
My Staff says I never listen to them, or something like that
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